The use of silica/silane filler systems to reduce the rolling resistance and improve the wet traction of passenger car tires is known in the art. A reduction of rolling resistance and, therefore, less fuel consumption also is of import for truck tires.
The simultaneous improvement of rolling resistance, wear and traction, known as expanding the “magic triangle”, requires new approaches to rubber composite development. Precipitated silica has played a major role in the emergence of the green tire, which boasts a large improvement in rolling resistance compared to past technologies. The direct cross-linking of silica into a highly cross-linked polymer matrix, while minimizing interactions between silica particles, is believed to be of vital importance to desirable dynamic mechanical properties. In natural rubber, proteins present from its biosynthesis can preferentially adsorb to the silica surface, interfering with the in-situ coupling reaction. Furthermore, increased dump temperatures, which may improve the coupling efficiency, have also been shown to degrade natural rubber.
The U.S. Environmental Protection Agency (EPA), in partnership with the U.S. Department of transportation (DOT) recently announced the first ever fuel economy and greenhouse gas emission standards for medium and heavy duty trucks. The benefits of filling passenger tire tread compounds made from styrene-butadiene rubber (SBR) and/or butadiene rubber (BR) with amorphous precipitated silica have been known as early as 1976. Unfortunately, silica/silane technology has not seen wide adoption in truck tires.
Truck tires are typically produced from natural rubber, which is a biosynthetic latex produced by the rubber tree, Hervia Brasiliensis. Solid natural rubber produced by coagulation typically contains some non-rubber components. These components may consist of proteins, lipids, sugars, ash and other impurities it has been proposed that traditional rubber processing methods, in which coupling agent and filler are added independently to the mixer, result in preferential adsorption of proteins onto the silica surface, thereby resulting in incomplete coupling of the silane moieties. Proteins have long been known to adsorb onto silica via strong intermolecular attractions. Further, it has been theorized that there is incomplete coupling of the silica functionality to polymer chain ends in natural rubber because the requisite number of highly reactive pendant 1,2-vinyl groups (which are present in varying amounts in synthetic polymers used in passenger tires) are not present in natural rubber.
Also, the elevated mixing temperatures (in some instances as high as 150° C.) for optimal in-situ coupling of silane to silica has previously been shown to degrade natural rubber.
Accordingly, further improvements in silica/silane technology are desired in order to overcome the above-mentioned deficiencies.